On 4/21/26 4:50 AM, Thomas Gleixner wrote:
On Mon, Apr 20 2026 at 23:03, Waiman Long wrote:
Full dynticks can only be enabled if "nohz_full" boot option has been
been specified with or without parameter. Any change in the list of
nohz_full CPUs have to be reflected in tick_nohz_full_mask. Introduce
a new tick_nohz_full_update_cpus() helper that can be called to update
the tick_nohz_full_mask at run time. The housekeeping_update() function
is modified to call the new helper when the HK_TYPE_KERNEL_NOSIE cpumask
is going to be changed.
We also need to enable CPU context tracking for those CPUs that
We need nothing. Use passive voice for change logs as requested in
documentation.
are in tick_nohz_full_mask. So remove __init from tick_nohz_init()
and ct_cpu_track_user() so that they be called later when an isolated
cpuset partition is being created. The __ro_after_init attribute is
taken away from context_tracking_key as well.
Also add a new ct_cpu_untrack_user() function to reverse the action of
ct_cpu_track_user() in case we need to disable the nohz_full mode of
a CPU.
With nohz_full enabled, the boot CPU (typically CPU 0) will be the
tick CPU which cannot be shut down easily. So the boot CPU should not
be used in an isolated cpuset partition.
With runtime modification of nohz_full CPUs, tick_do_timer_cpu can become
TICK_DO_TIMER_NONE. So remove the two TICK_DO_TIMER_NONE WARN_ON_ONCE()
checks in tick-sched.c to avoid unnecessary warnings.
in tick-sched.c? Describe the functions which contain that.
static inline void tick_nohz_task_switch(void)
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index 925999de1a28..394e432630a3 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -411,7 +411,7 @@ static __always_inline void ct_kernel_enter(bool user, int
offset) { }
#define CREATE_TRACE_POINTS
#include <trace/events/context_tracking.h>
-DEFINE_STATIC_KEY_FALSE_RO(context_tracking_key);
+DEFINE_STATIC_KEY_FALSE(context_tracking_key);
EXPORT_SYMBOL_GPL(context_tracking_key);
static noinstr bool context_tracking_recursion_enter(void)
@@ -674,9 +674,9 @@ void user_exit_callable(void)
}
NOKPROBE_SYMBOL(user_exit_callable);
-void __init ct_cpu_track_user(int cpu)
+void ct_cpu_track_user(int cpu)
{
- static __initdata bool initialized = false;
+ static bool initialized;
if (cpu == CONTEXT_TRACKING_FORCE_ENABLE) {
static_branch_inc(&context_tracking_key);
@@ -700,6 +700,15 @@ void __init ct_cpu_track_user(int cpu)
initialized = true;
}
+void ct_cpu_untrack_user(int cpu)
+{
+ if (!per_cpu(context_tracking.active, cpu))
+ return;
+
+ per_cpu(context_tracking.active, cpu) = false;
+ static_branch_dec(&context_tracking_key);
+}
+
Why is this in a patch which makes tick/nohz related changes? This is a
preparatory change, so make it that way and do not bury it inside
something else.
Sure, I will break out the context tracking change into its own patch.
+/* Get the new set of run-time nohz CPU list & update accordingly */
+void tick_nohz_full_update_cpus(struct cpumask *cpumask)
+{
+ int cpu;
+
+ if (!tick_nohz_full_running) {
+ pr_warn_once("Full dynticks cannot be enabled without the nohz_full
kernel boot parameter!\n");
That's the result of this enforced enable hackery. Make this work
properly.
+ return;
+ }
+
+ /*
+ * To properly enable/disable nohz_full dynticks for the affected CPUs,
+ * the new nohz_full CPUs have to be copied to tick_nohz_full_mask and
+ * ct_cpu_track_user/ct_cpu_untrack_user() will have to be called
+ * for those CPUs that have their states changed. Those CPUs should be
+ * in an offline state.
+ */
+ for_each_cpu_andnot(cpu, cpumask, tick_nohz_full_mask) {
+ WARN_ON_ONCE(cpu_online(cpu));
+ ct_cpu_track_user(cpu);
+ cpumask_set_cpu(cpu, tick_nohz_full_mask);
+ }
+
+ for_each_cpu_andnot(cpu, tick_nohz_full_mask, cpumask) {
+ WARN_ON_ONCE(cpu_online(cpu));
+ ct_cpu_untrack_user(cpu);
+ cpumask_clear_cpu(cpu, tick_nohz_full_mask);
+ }
+}
So this writes to tick_nohz_full_mask while other CPUs can access
it. That's just wrong and I'm not at all interested in the resulting
KCSAN warnings.
tick_nohz_full_mask needs to become a RCU protected pointer, which is
updated once the new mask is established in a separately allocated one.
I will work on that.
Cheers,
Longman
Thanks,
tglx
